A traditionally known two-cycle engine includes: a scavenging passage that includes a main passage provided near an exhaust port for combustion gas and a sub passage provided near an intake port of air-fuel mixture; and a throttle provided on the sub passage (see Document 1: JP-A-60-153428, for instance). The provision of the throttle on the sub passage of the two-cycle engine allows flow speed control in the sub passage by changing the size of the throttle, thus restraining blow-by of the air-fuel mixture during the scavenging process.
Since blow-by-restraining effect of the air-fuel mixture is greatly influenced by subtle flow speed change in the sub-passage of the two-cycle engine, the most appropriate size of the throttle is normally found by testing with various sizes of the throttle in order to control the flow speed in the sub-passage when the engine is manufactured.
However, since the throttle of the scavenging passage is provided by a thick portion of the cylinder wall or the thick portion of the crankcase, in order to vary the size of the throttle, the whole cylinder or the crankcase has to be re-produced while varying the thickness at the thick portion, which takes a lot of work and consequently makes it difficult to easily find the most appropriate value of the size of the throttle.